Most airports currently use an airport surveillance radar system which scans the skies to monitor incoming and outgoing aircraft. However, this surveillance radar has an altitude limit below which it cannot detect aircraft. Just before landing and immediately after takeoff, the aircraft are monitored visually or through radio communications directly with the pilot. To assist aircraft in landing when visibility is poor, many airports have installed an instrument landing system (ILS) and marker beacons. The ILS allows an airplane to determine whether it is on the proper flight path for a proper landing on the assigned runway. The marker beacons are spaced out at predetermined distances from the start of the runway and indicate to the aircraft the distance to the runway. The ILS and marker beacons however are used only to indicate the aircraft's position to the aircraft itself and do not assist the airport tower or air traffic controllers.